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1 Astronomical Notes Astronomische Nachrichten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stron. Nachr. / AN 329, No. 8, (28) / DOI 1.12/asna T N I R P E R

2 Astron. Nachr. / AN 329, No. 8, (28) / DOI 1.12/asna B photometry of a possible open star cluster pair NGC 731/. S. Kopchev and G. T. Petrov Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko chaussee Blvd, 1784 Sofia, Bulgaria Received 27 Feb 2, accepted 28 Aug 2 Published online 28 Sep 22 Key words open clusters and associations: individual (NGC 731, ) techniques: photometric We present a CCD B photometry of the possible binary open star cluster NGC 731/. The aim is to confirm or disprove their common nature on the grounds of their age and distance. An age of 224 ± 25 Myr and a distance of 831 ± 72 pc was determined for NGC 731, and 178 ± 25 Myr, 955 ± 84 pc for, respectively. Based on these differences in age and distance we conclude that the two clusters are most likely not formed together from one and the same giant molecular cloud and thus do not form a true binary cluster. 1 Introduction A binary open star cluster is an object consisting of two open clusters. They can be basically described as: (i) binary physical systems with common origin formed together from one and the same giant molecular cloud (GMC), having comparable ages and chemical compositions (we call this a true binary cluster); (ii) binary physical systems arising from clusters formed in different parts of the Galaxy and forming a pair through gravitational capture. Cluster pairs formed through tidal capture have different ages and/or chemical composition. The existence of star cluster pairs in our neighbouring galaxies, the Magellanic Clouds, was confirmed by several authors (e.g. Bhatia & Hatzidimitriou 1988; Bica et al.1992; allenari et al. 1998; Dieball & Grebel 2; de Oliveira et al. 2). Dieball (22) proposed a catalogue of binary and multiple cluster candidates in the Large Magellanic Cloud with 473 members. There are more than 16 open clusters in our Galaxy but only one well established double or binary cluster, namely h+χ Persei (see e.g. Uribe et al. 22; Kharchenko et al. 25). Our Galaxy seems to show a lack of binary or multiple clusters as compared to the Magellanic Clouds. It is still an open question why such objects do not exist in our Galaxy. The reasons could be that they have already merged or dissipated, or that they simply do not form. Several lists of binary open cluster candidates have been proposed and studied by various authors: Lyngå&Wramdemark (1984), Pavlovskaya & Filippova (1989), Tignanelli et al. (199), Subramaniam et al. (1995), Loktin (1997), and Muminov (2). One of the most complete and well studied lists is the one of Subramaniam et al. (1995) with 18 candidate pairs, including the clusters NGC 731 and studied here. Basic parameters of NGC 731 and Corresponding author: kopchev@astro.bas.bg Table Basic parameters of the clusters NGC 731 and NGC Parameter NGC 731 R.A. (2) 21:7:12 21:3:27 Dec. (2) +5:52:3 +51:36: Distance (pc) Ang. diam. (arcmin) E(B ) (mag) log(age/yr) as given in Dias et al. (22) and the WEBDA 1 database are presented in Table 1. Table 2 presents previous photographic studies of the open clusters NGC 731 and. The scatter, especially in the ages, is quite large for each cluster; all studies agree on a larger distance to in comparison with the distance proposed for NGC 731. For a good selection criterion for a binary cluster Dieball (22) considers: (i) the maximum centre-to-centre separation is 2 pc, and (ii) the age difference between the components of a binary cluster is either 1 Myr, or their ages agree well within the uncertainties of their age determination. Based on N-body simulations Portegies Zwart & Rusli (27) conclude that a cluster pair with a smaller initial separation tends to merge in 6 Myr due to loss of angular momentum from escaping stars, while clusters with a larger initial separation tend to become even more widely separated due to mass loss from the evolving stellar populations. This suggests that 2 pc is a good selection criterion. The aim of our investigation is to determine more precisely cluster parameters such as reddening, distance and 1

3 846. S. Kopchev & G. T. Petrov: B photometry of a possible open star cluster pair NGC 731/ Table 2 Previous studies of the open clusters NGC 731 and. Name Reference Distance Age E(B ) (pc) (Myr) (mag) NGC 731 Svolopoulos Hoag et al Lindoff Hassan & Barbone Ruprecht et al Janes & Adler Hassan Lindoff Janes & Adler Table 3 Observation log. Name Filter Exposure Airmass (s) NGC 731 B B Fig band image of NGC 731. North is to the top, East to the left. age using CCD photometry and applying criterion for binarity in order to confirm or disprove their binarity. 2 Observations and data reduction The clusters were observed in the night of 26 May 3 with the 2-m Ritchey-Chretien telescope of Rozhen National Astronomical Observatory, Bulgaria. The telescope is equipped with standard Johnson filters and ersarray 13B CCD camera with 2 μm pixel size that corresponds to.26 arcsec, giving a field of 5 6arcmin 2 in the sky. The angular separation between the centres of the clusters is approximately 1 and we cannot observe them on a single frame. The cluster fields are shown in Figs. 1 and 2. Table 3 presents an observation log, the seeing was between arcsec. Standard IRAF routines were used to reduce the data, photometry was carried out with DAOPHOT II. The accuracy of the photometry is given in Fig. 3 where colour and magnitude errors are plotted as a function of the magnitude. Instrumental magnitudes have been transformed to standard Johnson-Cousin system using a standard field around the cluster M92 (Majewski et al. 1994). The calibration equations we received are b = B (9 ±.6) + (.28 ± )X (.114 ±.6)(B ), v = (.737 ±.6) + (.16 ± )X (8 ±.6)(B ), where X is the airmass, the capital letters represent the standard magnitudes and colour and lower-case letters denote instrumental magnitudes. Fig band image of. North is to the top, East to the left. 3 Colour-magnitude diagrams and cluster parameters The luminosity function of the clusters is presented in Fig. 4 with a dashed line for NGC 731 and a solid line for NGC 786. Colour-magnitude diagrams (CMDs) are presented in Fig. 5 for NGC 731 and Fig. 6 for, where the solid line represents the zero age main sequence (ZAMS) taken from Schmidt-Kaler (1982). Both CMDs reveal the presence of a reasonably broad and slightly evolved main sequence (MS), typical of an early intermediate-age open

4 Astron. Nachr. / AN (28) 847 B- error B- error NGC error error NGC Fig. 3 The color and magnitude errors from DAOPHOT as a function of the magnitude. Fig. 4 Luminosity function of NGC 731 and. Table 4 Derived cluster parameters. Parameter NGC 731 E(B ) (mag) 1.5 ±.75 ± (m M) (mag) 9.6 ± ±.2 Distance (pc) 831 ± ± 84 log(age/yr) age (Myr) 224 ± ± 25 cluster. The MS extends over a range of 6 magnitudes, completeness limits are around =15. To estimate the field star contamination (see, e.g. Bonatto & Bica 28) we used the Besancon model of stellar population 2. The simulations determine how many field stars can be expected in the fields of view of the clusters. For cluster NGC 731 we do not expect field star contamination in the range =11 12,but for the simulation showed one field star in the range =1 11. Distance module, reddenings and ages of the clusters have been derived by matching by eye the observed CMDs to isochrones with Z = from the Geneva group (Schaller et al. 1992), paying particular attention to the most likely shape of the main sequence, the turn-off point and the location of the evolved stars. In Table 4 we present our determination of cluster parameters. 2 Fig. 5 The observed,(b ) diagram of NGC 731 and the best isochrones fit with log(age) =8.35 (solid curve). 4 Summary and conclusion We have presented B CCD photometry of the closely projected open star clusters NGC 731 and. The results have been summarized in Table 4. Our estimations of the age difference between NGC 731 and, and difference in distance along the line of sight are 46 Myr and 124 pc, respectively. These results do not match with the criterion for a binary cluster and based on these results we conclude that the two clusters are most likely not formed towww.an-journal.org

5 848. S. Kopchev & G. T. Petrov: B photometry of a possible open star cluster pair NGC 731/ References Fig. 6 The observed,(b ) diagram of and the best isochrones fit with log(age) =8.25 (solid curve). gether from one and the same GMC and they are not a true binary cluster. However, the ages of the two clusters roughly agree within the errors, and if we assume a mean distance of 893 pc and an angular separation of 1, this corresponds to a separation of 15 pc between the two clusters. These results are close enough for a true binary cluster. We caution that the distance differences seem to indicate that the cluster separation is probably larger than 12 pc, thus it seems unlikely the two clusters form a true binary cluster, but it cannot be completely ruled out. Follow-up observations to determine the radial velocities of the clusters members and the motion of the clusters itself could put more light to this problem. Bhatia, R.K., Hatzidimitriou, D.: 1988, MNRAS 23, 215 Bica, E., Clariá, J.J., Dottori, H.: 1992, AJ 13, 1859 Bonatto, C., Bica, E.: 28, A&Ap 479, 741 de Oliveira, M.R., Dutra, C.M., Bica, E., Dottori, H.: 2, A&AS 146, 57 Dias, W.S., Alessi, B.S., Moitinho, A., Lèpine, J.R.D.: 22, A&A 389, 871 Dieball, A.: 22, PhD Thesis Dieball, A., Grebel, E.K.: 2, A&A 358, 897 Hassan, S.M.: 1967, ZA 66, 6 Hassan, S.H., Barbon, R.: 1973, MmSAI 44, 39 Hoag, A.A., Johnson, H.L., Iriarte, B., Mitchell, R.I., Hallam, K.L., Sharpless, S.: 1961, Publ. US. Nav. Obs. 17, 347 Janes, K., Adler, D.: 1982, ApJS 49, 425 Kharchenko, N.., Piskunov, A.E., Röser, S., Schilbach, E., Scholz, R.-D.: 25, A&Ap 438, 1163 Lindoff, U.: 1968, ArA 5, 1 Loktin, A..: 1997, A&AT 14, 181 Lyngå, G., Wramdemark, S.: 1984, A&A 132, 58 Majewski, S.R., Kron, R.G., Koo, D.C., Bershady, M.A.: 1994, PASP 16, 1258 Muminov, M.: 2, AGM 16poster 7 Pavlovskaya, E.D., Filippova, A.A.: 1989, SvA 33, 62 Portegies Zwart, S.F., Rusli, S.P.: 27, MNRAS 374, 931 Ruprecht, J., Balazs, B., White, R.: 1981, Catalogue of Star Clusters and Associations, Akademiai Kiado, Budapest Schaller, G., Schaerer, D., Meynet, G., Maeder, A.: 1992, A&AS 96, 269 Schmidt-Kaler, T.S.: 1982, in: K. Schaifers, H.H. oigt (eds.), Landolt-Börnstein, New Series, Group I, ol. 2b, p. 1 Subramaniam, A., Gorti, U., Sagar, R., Bhatt, H.C.: 1995, A&A 32, 86 Svolopoulos, S.N.: 1961, ApJ 134, 612 Tignanelli, H., azquez, R.A., Mostaccio, C., Gordillo, S., Feinstein, A., Plastino A.: 199, RMxAA 21, 35 Uribe, A., Garcia-arela, J.-A., Sabogal-Martinez B.-E., Higuera, G.M.A., Brieva, E.: 22, PASP 114, 233 alenari, C., Bettoni, D., Chiosi, C.: 1998, A&A 331, 56 Acknowledgements. This research has made use of the WEBDA database, operated at the Institute for Astronomy of the University of ienna.